Process for manufacturing alkanes with methane

ABSTRACT

The present invention relates to a process for manufacturing alkanes. comprising, as main stage, a reaction resulting from bringing methane into contact with at least one other starting alkane (A) in the presence of a catalyst based on a metal M capable of catalysing a metathesis of alkanes. The reaction results in the formation of at least one or two final alkanes (B) having a number of carbon atoms less than or equal to that of the starting alkane (A) and at least equal to 2. Preferably the catalyst comprises a hydride of a metal M grafted to and dispersed over a solid support. The metal M may be chosen from transition metals, lanthanides and actinides. The present invention also relates to the use of a catalyst capable of catalysing a metathesis of alkanes in a reaction resulting from bringing methane into contact with at least one other starting alkane (A).

[0001] The present invention relates to a process for the manufacture ofalkanes by a catalytic reaction employing methane with at least oneother alkane.

[0002] Alkanes, such as methane, are generally products which aredifficult to employ because of their chemical inertia. Nevertheless, theconversion of alkanes into other alkanes is known. Hydrogenolysisreactions, which consist of cleavage or opening reactions of acarbon-carbon bond by hydrogen, are known, for example. Isomerizationreactions, which convert an alkane into one of its isomers, for examplen-butane into isobutane, are also known. All these reactions aregenerally carried out at relatively high temperatures and in thepresence of catalysts based on metals, in particular on transitionmetals, in the bulk form or in the form of films or alternatively in theform of metal particles deposited on inorganic supports essentiallybased on metal oxide or refractory oxide. Thus, for example, thecatalyst can be of the following types: nickel black, Ni/SiO₂, platinumblack Pt/SiO₂, Pd/Al₂O₃, or tungsten or rhodium film, optionally mixedwith copper, tin or silver. With some metal catalysts, it was possiblesimultaneously to observe alkane homologation reactions, which consistof reactions which convert alkanes into higher homologous alkanes.However, alkane homologation reactions are generally very minorreactions in comparison with the hydrogenolysis or isomerizationreactions and their results are very poor.

[0003] Nevertheless, it remains the case that a process for theconversion of an alkane into one of its homologues would constitute ameans for enhancing these alkanes in value, in particular methane. It isknown that, as a general rule, alkanes of low molecular weight cannot beexploited to any great extent in chemistry or petrochemistry, other thanas fuels, whereas heavier alkanes are often of greater commercialinterest, such as, for example, to increase the octane number of enginefuels or alternatively to involve these heavier alkanes in thermal orthermal catalytic cracking or steam cracking reactions in order tomanufacture, for example, olefins or dienes.

[0004] In this sense, Patent Application PCT/FR 97/01266 discloses aprocess for the metathesis of alkanes. A metathesis is a doubledecomposition reaction of two identical or different compounds whichforms two new compounds by a double recombination. In this case, atleast one alkane is reacted with itself or several alkanes with oneanother in the presence of a solid catalyst comprising a metal hydridegrafted to and dispersed over a solid oxide. Thus, the metathesisreaction is carried out in the presence of this metal hydride bycleavage and recombination of the carbon-carbon bonds, converting analkane simultaneously into its higher and lower homologues. The reactioncan be written according to the following equation (1):

2C_(n)H_(2n+2)→C_(n−i)H_(2(n−i)+2)C_(n+i)H_(2(n+i)+2)   (1)

[0005] where i=1, 2, 3, . . . n−1 and n can range from 2 to 30 and evenbeyond.

[0006] The catalyst used is a catalyst based on metal hydride andcomprises a transition metal chosen in particular from those from groups5 and 6 of the Table of the Periodic Classification of the Elements (asdefined by IUPAC in 1991 and illustrated in “Hawley's Condensed ChemicalDictionary”, 12^(th) edition, by Richard J. Lewis, Sr., published by VanNostrand Reinhold Company, New York, 1993), such as, in particular,tantalum, chromium or tungsten. The preparation of the catalystcomprises a stage of hydrogenation of an organometallic precursorcomprising a transition metal dispersed over and grafted to a solidoxide beforehand, so that the transition metal is reduced to anoxidation state lower than its maximum value, thus resulting in themetal hydride. However, as in any metathesis of alkanes, in particularcarried out in the presence of this metal hydride, higher and lowerhomologous alkanes are simultaneously manufactured, by cleavage andrecombination reactions of carbon-carbon bonds, employing at least C₂alkanes (ethane).

[0007] A novel process for the manufacture of alkanes has now been foundwhich makes use of a reaction resulting from bringing methane intocontact with at least one other starting alkane in the presence of acatalyst capable of catalysing a metathesis of alkanes. The process hasthe advantage of enhancing the value of methane, which is available inlarge amounts on the market and is known for being used essentially as afuel. Finally, the process makes possible the direct manufacture of thedesired product without forming a large number of by-products and thusmakes it possible to avoid or to cut back on lengthy and expensiveoperations for the separation and isolation of the desired product.

[0008] A subject-matter of the invention is therefore a process for themanufacture of alkanes, characterized in that it comprises, as mainstage, a reaction resulting from bringing methane into contact with atleast one other starting alkane (A) in the presence of a catalyst basedon a metal M capable of catalysing a metathesis of alkanes, whichreaction results in the formation of at least one or two final alkanes(B) having a number of carbon atoms less than or equal to that of thestarting alkane (A) and at least equal to 2.

[0009] More particularly, a catalytic reaction is carried out whichresults from bringing methane into contact with at least one otherstarting C_(n) alkane (A) (that is to say, comprising n carbon atoms),with n being equal to at least 2, preferably to at least 3, so that thereaction results in the formation of at least one or two final C₂ toC_(n) alkanes (B) (that is to say, having a number of carbon atomsranging from 2 to n).

[0010] The reaction can be written according to one or more of thefollowing equations (2):

[0011] in which equation (2) n is an integer at least equal to 2,preferably at least equal to 3, and a is an integer ranging from 1 ton−1.

[0012] Thus, the process of the invention comprises, as main stage, oneor more reactions resulting from bringing methane into contact with atleast one other starting alkane (A), the mechanisms of which reactionshave not yet been clearly determined. This is because it is particularlysurprising to find that methane, which does not comprise a carbon-carbonbond, can react directly or indirectly with another starting alkane (A)in the presence of a catalyst capable of catalysing a reaction for themetathesis of alkanes by cleavage and recombination of the carbon-carbonbonds. The reaction employed in the process of the present invention iscarried out by simply bringing methane into contact with at least oneother starting alkane (A) in the presence of a catalyst for themetathesis of alkanes and under relatively mild conditions, as describeda little later.

[0013] The starting alkane (A) can be a substituted or unsubstitutedacyclic alkane, that is to say composed of a linear or branched butunclosed carbon-comprising chain. It can correspond to the generalformula:

C_(n)H_(2n+2)   (3)

[0014] in which n is an integer ranging from 2 to 60 or from 3 to 60,preferably from 3 to 50, in particular from 3 to 20.

[0015] The starting alkane (A) can also be a cyclic alkane orcycloalkane substituted in particular by a linear or branchedcarbon-comprising chain, for example by an alkyl radical. It cancorrespond to the general formula:

C_(n)H_(2n)   (4)

[0016] in which n is an integer ranging from 5 to 60, preferably from 5to 20, in particular from 5 to 10.

[0017] Use may be made of one or more starting alkanes (A) such as thosedescribed above.

[0018] More particularly, the starting alkane (A) can be chosen from C₃to C₁₀ or C₃ to C₁₇ alkanes, for example propane, n-butane, isobutane,n-pentane, isopentane, n-hexane, n-heptane, n-octane, n-nonane andn-decane.

[0019] Thus, for example, in the process of the present invention,methane can be brought into contact with propane to form ethane oralternatively methane can be brought into contact with n-butane ethaneand propane.

[0020] The staring alkane (A) can also be chosen from paraffins, such asn-paraffins, isoparaffins and cycloparaffins, for example C₁₈ to C₆₀ orC₂₂ to C₆₀ or alternatively C₂₂ to C₄₅ n-paraffins, isoparaffins andcycloparaffins.

[0021] Methane is brought into contact with at least one other startingalkane (A) in the presence of a catalyst based on a metal M capable ofor known for catalysing a metathesis of alkanes. It is in particular acatalyst which, if it were brought into contact with at least onealkane, for example a C₂ to C₃₀ alkane, would result in a metathesis ofthe alkane as represented by the equation (1). It can in particular be acatalyst comprising a hydride of a metal M grafted to and dispersed overa solid support, such as a metal oxide or sulphide or refractory oxideor sulphide Without it being possible to explain in detail the catalyticmechanism of the main reaction of the process of the present invention,it is likely to imagine the catalyst as acting as a catalyticintermediate When it is brought into contact with methane, the catalystcan probably form a methyl-metal M complex, which might be thecatalytically active species with regard to the starting alkane (A).

[0022] The catalyst comprises, for example, a solid support to which aregrafted and over which are dispersed metal atoms of the metal M whichare found in the hydride form. Thus, the catalyst preferably comprises ametal M bonded to at least one hydrogen atom.

[0023] The metal M can be chosen from transition metals, in particularthe metals from columns 3, 4, 5 and 6 of the Table of the PeriodicClassification of the Elements mentioned above, and from lanthanides andactinides. The metal can, for example, be chosen from scandium, yttrium,lanthanum, titanium, zirconium, hafnium, vanadium, niobium, tantalum,chromium, molybdenum, tungsten, cerium and neodymium. Preference isgiven to a metal chosen from the transition metals of the abovementionedcolumns 4, 5 and 6 and in particular from titanium, zirconium, hafnium,vanadium, niobium tantalum, chromium, molybdenum and tungsten. Moreparticularly, preference is given to tantalum, chromium, vanadium,niobium, molybdenum or tungsten.

[0024] The metal M present in the catalyst in the hydride form andattached to the solid support is generally at an oxidation state lowerthan its maximum value. It can, for example, be at an oxidation statelower by 1 or 2 points than its maximum value. In particular, the metalcan be in a state of advanced electronic unsaturation: its valence layercan be highly deficient in electrons (less than 16 electrons); in thecases observed, there are approximately 10 electrons.

[0025] The metal hydride is attached to a solid support which can bechosen from oxides or sulphides. Preference is given to a solid support,such as a metal oxide or refractory oxide or a mixture of oxides, forexample silica, alumina, a mixture of silica and alumina, zeolites,natural clays, aluminium silicates, titanium oxide, magnesium oxide,niobium oxide or zirconium oxide. The solid support can be a metal oxideor refractory oxide modified by an acid, such as a sulphated zirconia ora sulphated alumina. The solid support can also be a metal sulphide,such as a molybdenum or tungsten sulphide, a sulphurized alumina or asulphurized metal oxide. It is preferable to use a solid support chosenfrom silicas and aluminas, in particular porous or non-porous silicasand aluminas, for example mesoporous silicas and aluminas having poresof 20 to 200 Å.

[0026] The solid support based on metal oxide or refractory oxide hasthe advantage of exhibiting, at its surface, oxygen atoms which can formpart of the coordination shell of the metal M. Thus, the metal M canadvantageously be bonded to one or, preferably, to at least twofunctional groups of the solid support. In this case, if the solidsupport is a metal oxide or a refractory oxide, the metal can be bondedto one or, preferably, to at least two oxygen atoms of the solidsupport. The presence of one or, preferably, of at least twooxygen-metal bonds confers greater stability on the metal hydride whileproviding a strong support-metal bond.

[0027] The catalyst described above can be prepared in various ways. Oneof the preparation processes can comprise the following two stages:

[0028] (a) the dispersion over and the grafting to the solid support ofan organometallic precursor (P) comprising the metal M bonded to atleast one hydrocarbon-comprising ligand, then

[0029] (b) the treatment of the product resulting from the precedingstage with hydrogen or a reducing agent capable of forming a metalM-hydrogen bond, in particular by hydrogenolysis of thehydrocarbon-comprising ligands.

[0030] The organometallic precursor (P) comprises the metal M describedabove bonded to at least one hydrocarbon-comprising ligand. It cancorrespond to the general formula

MR   (5)

[0031] in which M represents the metal of the catalyst as describedabove, R represents one or more identical or different, saturated orunsaturated, hydrocarbon-comprising ligands, in particular aliphatic oralicyclic hydrocarbon-comprising ligands, preferably from C₁ to C₂₀,especially from C₁ to C₁₀, and a is an integer equal to the oxidationstate of the metal M.

[0032] The metal M in the organometallic precursor (A) can be at anoxidation state lower than or, preferably, equal to its maximum value.

[0033] The metal M can be bonded to one or more carbons of thehydrocarbon-comprising ligands R via one or more carbon-metal single,double or triple bonds. It can be in particular a carbon-metal singlebond of a type: in this case, the hydrocarbon-comprising ligand is analkyl radical, for example a linear or branched alkyl radical. The term“alkyl radical” is understood to mean a monovalent aliphatic radicaloriginating from the removal of a hydrogen atom in the molecule of analkane or of an alkane or of an alkyne, for example a methyl (CH₃—),ethyl (C₂H₅—), propyl (C₂H₅—CH₂—), neopentyl ((CH₃)₃C—CH₂—), allyl(CH₂═CH—CH₂—) or acetylene (CH≡C—) radical. The alkyl radical can be,for example, of formula R—HC₂—, where R itself represents a linear orbranched alkyl radical.

[0034] It can also be a carbon-metal double bond of π type: in thiscase, the hydrocarbon-comprising ligand is an alkylidene radical, forexample a linear or branched alkylidene radical. The term “alkylideneradical” is understood to mean a bivalent aliphatic radical originatingfrom the removal of two hydrogen atoms on the same carbon of themolecule of an alkane or of an alkane or of an alkyne, for example amethylidene (CH₂═), ethylidene (CH₃—CH═), propylidene (C₂H₅—CH═),neopentylidene ((CH₃)₃C—CH═) or alkylidene ((CH₂═CH—CH═) radical. Thealkylidene radical can be, for example, of formula R—CH═, where Rrepresents a linear or branched alkyl radical

[0035] The carbon-metal bond can also be a triple bond: in this case,the hydrocarbon-comprising ligand is an alkylidyne radical, for examplea linear or branched alkylidyne radical The term “alkylidyne radical” isunderstood to mean a trivalent aliphatic radical originating from theremoval of three hydrogen atoms on the same carbon of the molecule of analkane or of an alkane or of an alkyne, for example an ethylidyne(CH₃—C≡), propylidyne (C₂H₅—C≡), neopentylidyne ((CH₃)₃C—C≡) oralkylidyne (CH₂ 50 CH—C≡) radical. The alkylidyne radical can be, forexample, of formula R—C≡, where R represents a linear or branched alkylradical. It is preferable to have, among the alkyl, alkylidene andalkylidyne radicals, in particular methyl, ethyl, propyl, isobutyl,neopentyl, allyl, neopentylidene, alkylidene and neopentylidyneradicals.

[0036] The metal M of the organometallic precursor (P) can be bonded totwo or more identical or different hydrocarbon-comprising ligands chosenfrom alkyl alkylidene and alkylidyne radicals. In particular, it can bebonded to at least one alkyl radical and to at least one alkylidene oralkylidyne radical.

[0037] The preparation of the catalyst comprises a first stage duringwhich the organometallic precursor (P) is dispersed over and grafted toa solid support, as described above. The support, which is preferably ametal oxide or refractory oxide, such as silica, is subjected to a heattreatment which is capable in particular of bringing about a dehydrationand/or a dehydroxylation, in particular between 200 and 1100° C., forseveral hours, for example from 2 to 48 hours, preferably from 10 to 24hours. The maximum temperature of the heat treatment is preferably belowthe sintering temperature of the solid support. Thus, for a silica, adehydration and/or a dehydroxylation can be carried out at a temperatureof 200 to 500° C., for example of 300 to 500° C., or else at atemperature ranging from 500° C. to the sintering temperature of thesilica, in order in particular to form siloxane bridges at the surfaceof the support.

[0038] The operations of dispersing of the organometallic precursor (P)over the solid support and of grafting the organometallic precursor (P)to the solid support can be carried out by sublimation or by bringinginto contact in liquid medium or in solution.

[0039] In the case of a sublimation operation, the organometallicprecursor (P), used in the solid state, is heated under vacuum and undertemperature and pressure conditions which provide for its sublimationand its migration in the vapour state onto the support. The latter ispreferably used in pulverulent form or in the form of pellets. Thesublimation is carried out in particular between 25 and 300° C.,preferably between 50 and 150° C., under vacuum. In particular, thegrafting of the organometallic precursor (P) to the support can bemonitored using infrared spectroscopic analysis.

[0040] In the method which has just been described, the sublimation canbe replaced by an operation of bringing into contact and a reaction inliquid or solvent medium. In this case, the organometallic precursor (P)is preferably dissolved in an organic solvent, such as pentane or ether.The reaction is then carried out by suspending the support, preferablyin a pulverulent form, in the solution comprising the organometallicprecursor (P) or alternatively by any other method which providescontact between the support and the organometallic precursor (P). Thereaction can be carried out at room temperature (20° C.) or moregenerally at a temperature ranging from −80° C. to 150° C. under aninert atmosphere, for example a nitrogen atmosphere.

[0041] The excess organometallic precursor (P) which is not attached tothe support can be removed, for example by washing or reversesublimation.

[0042] The preparation of the catalyst subsequently comprises a secondstage during which the organometallic precursor, dispersed over andgrafted to the solid support, is brought into contact with hydrogen or areducing agent capable of converting the atoms of the metal M to metalhydrides, in particular by hydrogenolysis of the hydrocarbon-comprisingligands bonded to the metal. It is generally a reduction reaction on themetal M attached to the support, which thus has its oxidation statereduced to a value lower than its maximum value. The reaction can takeplace under an absolute pressure ranging from 10⁻³ to 10 MPa and at atemperature ranging from 25 to 400° C., preferably from 100 to 300° C.The reaction can be carried out over a period of time ranging from 1 to24 h, preferably from 10 to 20 h.

[0043] The catalyst can be prepared by other methods using otherprecursors, in so far as they result in a metal hydride of the metal Mwhich is supported and which is capable of catalysing an alkanemetathesis.

[0044] Mention may be made, among the preferred catalysts, of tantalum,tungsten or chromium hydrides which are grafted to and dispersed over asilica or a silica/alumina

[0045] Another subject-matter of the present invention is the use of acatalyst capable of catalysing a metathesis of alkanes in a reactionresulting from bringing methane into contact with at least one otherstarting alkane (A) under conditions which result in the formation of atleast one or two final alkanes (B) having a number of carbon atoms lessthan or equal to that of the starting alkane (A) but at least equal to2.

[0046] The process according to the invention can be carried outbatchwise or continuously. It can be carried out in the gas phase, inparticular in a mechanically stirred and/or fluidized bed reactor or ina stationary or circulating bed reactor, the bed being composedessentially of the catalyst. The process can also be carried out in theliquid phase, for example in the starting alkane (A) in the liquidstate, the catalyst being suspended in the liquid phase.

[0047] The process can be carried out in the presence of an inert,liquid or gaseous, agent, such as nitrogen, helium or argon.

[0048] The process can be carried out at a temperature ranging from −30to +400° C., preferably from 0 to 300° C., in particular from 20 to 200°C., under an absolute pressure ranging from 10⁻³ to 30 Mpa, preferablyfrom 10⁻¹ to 20 MPa, in particular from 10⁻¹ to 10 MPa.

[0049] In the process according to the invention, the methane and thestarting alkane(s) (A) can be added to the catalyst separately and inany order, or simultaneously by at least two separate introductions, oralternatively premixed and using a single introduction. The methane andthe starting alkane(s) (A) can be used in a (methane:starting alkane(s)(A)) molar ratio ranging from 0.1:1 to 500:1, preferably from 1:1 to200:1, in particular from 1:1 to 100:1.

[0050] The proportion of catalyst present in the reaction mixturecomposed of methane and the starting alkane(s) (A) can be such that themolar ratio of methane to the metal M of the catalyst is from 10:1 to10⁵:1, preferably from 50:1 to 10⁴:1, in particular from 50:1 to 10³:1.

[0051] The examples which follow illustrate the present invention.

EXAMPLE 1 Preparation of a Catalyst Based on Supported Tantalum Hydride

[0052] A catalyst based on supported tantalum hydride [Ta]_(s)—H isprepared in the following way: tris(neopentyl)neopentylidenetantalum ofgeneral formula

Ta[—CH₂—CMe₃]₃[═CH—CMe₃]

[0053] (in which Me represents the methyl radical) is sublimed at 80° C.in a glass reactor over a silica dehydroxylated beforehand at 500° C.and is then grafted by a reaction at 25° C. with the surface hydroxylgroups of the silica, which reaction corresponds to the followingequation (6):

3≡SiOH+2Ta[—CH₂—CMe₃]₃[═CH—CMe₃]→

≡SiO—Ta[—CH₂—CMe₃]₂[═CH—CMe₃]+(—SiO)₂—Ta[—CH₂—CMe₃][═CH—CMe₃]+3CMe₄  (6)

[0054] The mixture of the neopentylneopentylidenetantalum compoundswhich are thus obtained, which are dispersed over and grafted to silica:

≡SiO—Ta[—CH₂—CMe₃]₂[═CH—CMe₃]

[0055] and

(≡SiO)₂—Ta[—CH₂—CMe₃][═CH—CMe₃]

[0056] is subsequently treated under hydrogen at atmospheric pressure at150° C. for 15 h, so as to form supported tantalum hydride species byhydrogenolysis of the neopentyl and neopentylidene ligands.

EXAMPLE 2 Preparation of a Catalyst Based on Supported Tantalum Hydride

[0057] A catalyst based on supported tantalum hydride [Ta]_(s)—H isprepared in the following way: a silica is dehydroxylated beforehand ata temperature of 500° C. and then at 1100° C., so as to bring about theappearance at the surface of more or less strained siloxane bridgesresulting from the condensation of the hydroxyl groups;tris(neopentyl)neopentylidenetantalum of general formula

Ta[—CH₂—CMe₃]₃[═CH—CMe₃]

[0058] is sublimed at 80° C. and reacts with the residual hydroxylgroups and the siloxane bridges according to the following equation (7):

[0059] Conversion of the neopentylneopentylidene-tantalum compounds,dispersed over and grafted to silica, to supported tantalum hydrides iscarried out as in Example 1 by treatment under hydrogen.

EXAMPLE 3 Preparation of a Catalyst Based on Supported Tungsten Hydride

[0060] A catalyst based on supported tungsten hydride [W]_(s)—H isprepared in the following way: tris(neopentyl)neopentylidynetungsten ofgeneral formula

W[—CH₂—CMe₃]₃[≡C—CMe₃]

[0061] is sublimed at 80° C. in a glass reactor over a silicadehydroxylated beforehand at 500° C. and is then grafted by a reactionat 25° C. with the surface hydroxyl groups of the silica. The mixture ofthe tungsten compounds which are thus obtained and supported issubsequently treated under hydrogen at atmospheric pressure at 150° C.for 15 h, so as to form supported hydride species by hydrogenolysis ofthe neopentyl and neopentylidyne ligands.

EXAMPLE 4 Reaction of Methane with Ethane

[0062] The tantalum hydride supported on silica [Ta]_(s)—H catalyst (50mg; content by weight of tantalum =4.89% Ta/SiO₂; that is to say, 14micromol of tantalum) prepared in Example 1 is used.

[0063] A reactor with a capacity of 0.28 l comprising the abovementionedcatalyst is placed under vacuum, is then filled with a mixture of¹³C-labelled methane and of ethane (C₂) (unlabelled) with the followingpartial pressures (pp):

[0064] methane pp (¹³C-labelled)=64.5 kPa

[0065] ethane pp=1.2 kPa

[0066] and is heated at 165° C. under steady state conditions. Thereaction products are measured over time under these conditions and areanalysed by gas chromatography, optionally coupled with massspectrometry The results are collated in Table 1. TABLE 1 results of thereaction between ¹³C-labelled methane and ethane (C₂) Molar ratio of the¹³C incorporated in ethane to Time (h) % C2 % C2* % C2** the tantalum1.5 94 6 — 0.47 12 90 10 — 0.73 36 87 13 — 0.88 60 80 20 — 1.13 120 6728 5 1.99

[0067] The reactions involved in this example as main stage are asfollows:

¹³CH₄+CH₃—CH₃→C₄+¹³CH₃—CH₃

¹³CH₄+¹³CH₃—CH₃→CH₄+¹³CH₃—¹³CH₃

[0068] It is observed, from Table 1, that the carbon-13 of the methaneis gradually incorporated in the ethane molecule, which first becomessingly labelled, then doubly labelled, thereby showing a reactionbetween the methane and the ethane.

[0069] In addition to this main stage, other reactions take place inparallel by conventional metathesis reactions on the labelled orunlabelled ethane according to the equations (1), to form in particularpropane, in particular ¹³C-labelled propane.

EXAMPLE 5 Reaction of Methane with Ethane

[0070] The reaction is carried out exactly as in Example 4, except thatuse is made of the catalyst prepared in Example 2 (40 mg; content byweight of tantalum=4.89% Ta/SiO₂).

[0071] A gradual formation of singly ¹³C-labelled ethane, then doubly¹³C-labelled ethane, is observed, as in Example 4.

EXAMPLE 6 Reaction of Methane with Ethane

[0072] The reaction is carried out exactly as in Example 4, except thatuse is made of the catalyst prepared in Example 3 (53 mg; content byweight of tungsten=4.96% W/SiO₂).

[0073] A gradual formation of singly and then doubly ¹³C-labelled ethaneis observed, as in Example 4.

EXAMPLE 7 Reaction of Methane with Propane

[0074] The reaction is carried out exactly as in Example 4, except thatethane is replaced with propane.

[0075] It is observed that ethane progressively labelled with ¹³C isformed as main stage. In addition to this, other higher alkanes areprogressively formed by reactions according to the equation (1).

EXAMPLE 8 Reaction of Methane with n-Butane

[0076] The reaction is carried out exactly as in Example 4, except thatethane is replaced with n-butane.

[0077] It is observed that ethane and propane, both progressivelylabelled with ¹³C, are simultaneously formed as main stage. Other higheralkanes appear by reactions according to the equation (1).

1. Process for the manufacture of alkanes, characterized in that itcomprises, as main stage, a reaction resulting from bringing methaneinto contact with at least one other starting alkane (A) in the presenceof a catalyst based on a metal M capable of catalysing a metathesis ofalkanes, which reaction results in the formation of at least one or twofinal alkanes (B) having a number of carbon atoms less than or equal tothat of the starting alkane (A) and at least equal to
 2. 2. Processaccording to claim 1, characterized in that the starting alkane (A) ischosen from substituted or unsubstituted acyclic alkanes and substitutedcyclic alkanes.
 3. Process according to claim 1 or 2, characterized inthat the starting alkane (A) corresponds to the general formulaC_(n)H_(2n+2) in which n is an integer ranging from 2 to
 60. 4. Processaccording to claim 1 or 2, characterized in that the starting alkane (A)is a cycloalkane which is substituted and which corresponds to thegeneral formula C_(n)H_(2n) in which n is an integer ranging from 5 to60.
 5. Process according to any one of claims 1 to 3, characterized inthat the starting alkane (A) is chosen from propane, n-butane,isobutane, n-pentane, isopentane, n-hexane, n-octane, n-nonane andn-decane.
 6. Process according to any one of claims 1 to 3,characterized in that the starting alkane (A) is chosen from C₃ to C₁₇alkanes.
 7. Process according to any one of claims 1 to 3, characterizedin that the starting alkane (A) is chosen from C₁₈ to C₆₀ paraffins. 8.Process according to any one of claims 1 to 7, characterized in that thecatalyst comprises a hydride of a metal M grafted to and dispersed overa solid support.
 9. Process according to claim 8, characterized in thatthe metal M is chosen from transition metals, lanthanides and actinides.10. Process according to claim 9, characterized in that the metal M ischosen from titanium, zirconium, hafnium, vanadium, niobium, tantalum,chromium, molybdenum and tungsten
 11. Process according to any one ofclaims 8 to 10, characterized in that the metal M is at an oxidationstate lower than its maximum value.
 12. Process according to any one ofclaims 8 to 11, characterized in that the solid support is chosen frommetal oxides or refractory oxides.
 13. Process according to claim 12,characterized in that the metal M is bonded to one or, preferably, to atleast two oxygen atoms of the solid support.
 14. Process according toany one of claims 8 to 13, characterized in that the catalyst isprepared in two stages: (a) by dispersing over and grafting to the solidsupport an organometallic precursor (P) comprising the metal M bonded toat least one hydrocarbon-comprising ligand, then (b) by treating thesolid product resulting from the preceding stage with hydrogen or areducing agent capable of forming a metal M-hydrogen bond.
 15. Processaccording to any one of claims 1 to 14, characterized in that thereaction resulting from bringing methane into contact with at least theother starting alkane (A) is carried out at a temperature of −30 to+400° C. under an absolute pressure of 10⁻³ to 30 MPa.
 16. Processaccording to any one of claims 1 to 15, characterized in that thereaction resulting from bringing methane into contact with at least theother starting alkane (A) is carried out in the gas phase in amechanically stirred and/or fluidized bed reactor or in a stationary orcirculating bed reactor, the bed being composed essentially of thecatalyst.
 17. Process according to any one of claims 1 to 15,characterized in that the reaction resulting from bringing methane intocontact with at least the other starting alkane (A) is carried out inthe liquid phase, the catalyst being suspended in the liquid phase. 18.Process according to any one of claims 1 to 17, characterized in thatthe methane and the starting alkane(s) (A) are used in a(methane:starting alkane(s) (A)) molar ratio ranging from 0.1:1 to500:1.
 19. Process according to any one of claims 1 to 18, characterizedin that the catalyst is present in the reaction mixture composed ofmethane and at least the other starting alkane (A) in a proportion suchthat the molar ratio of methane to the metal M of the catalyst is from10:1 to 10⁵:1.
 20. Use of a catalyst capable of catalysing a metathesisof alkanes in a reaction resulting from bringing methane into contactwith at least one other starting alkane (A) under conditions resultingin the formation of at least one or two final alkanes (B) having anumber of carbon atoms less than or equal to that of the starting alkane(A) and at least equal to 2.